class Solution {
public:
    int climbStairs(int n) {
        int f_1 = 0;
        int f_2 = 0;
        int ret = 1;

        for (int i = 1 ;i<=n ; ++i)  {
            f_2 = f_1;
            f_1 = ret;
            ret = f_1 + f_2;
        }

        return ret;
    }
};